These days, various terminals such as a personal computer and a portable unit support IPv6, and IPv6 packets are already running in networks. Furthermore, currently, IPv6 addresses are the only available global addresses that can be newly acquired, which further rapidly accelerates a shift in networks to IPv6. This shift will further increase the number of environments where IPv4 and IPv6 coexist.
Patent Document 1 listed below describes data link layer (hereinafter, referred to as L2) address resolution and Neighbor Discovery Protocol (NDP) as functions of IPv6. In this NDP, states of each entry in a Neighbor Cache of each node are updated on the basis of a Neighbor Solicitation (NS) message and a Neighbor Advertisement (NA) message.
Furthermore, Patent Document 2 listed below proposes using a function of Duplicate Address Detection (DAD) of IPv6 to detect unauthorized access to a network where IPv4 and IPv6 coexist and block any terminal used for unauthorized access in this network.